The skip distance is the shortest distance from a transmitter, measured along the surface of the earth, at which a sky wave of fixed frequency(more than f) will be returned to earth. That there should be a minimum distance may come as a shock. One expects there to be a maximum distance, as limited by the curvature of the earth, but nevertheless a definite minimum also exists for any fixed transmitting frequency. The reason for this becomes apparent if the behavior of a sky wave is considered with the aid of a sketch, such as Figure 8-15.